Present borehole seismics involves techniques in which seismic signals generated at or near the surface of the earth or in a borehole are recorded by geophones secured at various depths to the wall of a borehole. Unlike the more commonly used land seismic or horizontal seismic profiling, where the geophones are strung along the earth surface, borehole seismics uses geophones at locations spaced along the borehole axis. These geophones are sensitive to velocity changes or acceleration and typically respond to both upgoing and downgoing seismic events, in contrast to horizontal seismic profiling, where the geophones typically cannot respond directly to downgoing events.
Borehole seismic measurements can give insight into some fundamental properties of propagating seismic waves and assist in the structural, stratigraphic, and lithological interpretation of subsurface formations. For example, an important use of vertical seismic profiling (VSP) measurements is to help define upgoing and downgoing seismic events within the earth and thereby help determine which events arriving at the surface are primary reflections and which are multiples. Other applications of borehole seismics include estimation of reflector dip, correlation of shear wave reflections with compressional wave reflections, location of fault planes, determination of lithological effects on propagating wavelets, looking for reflectors ahead of the drill bit, determining hydrocarbon effects on propagating wavelets, identification of intrabed multiples, measurement of both compressional and shear wave velocities, and estimation of the conversion of compressional to shear and shear to compressional energy modes within the earth. Background information concerning borehole seismics, in particular VSP can be found in Hardage, B. A., Vertical Seismic Profiling, Part A: Principles, Geophysical Press, 1983, Volume 14A of Handbook Of Geophysical Exploration, Section I. Seismic Exploration, Helbig and Treitel (Editors); Society of Exploration Geophysics, Expanded Abstracts of the Technical Program With Authors' Biographies, Sep. 11-15, 1983, Las Vegas, Nev., pp. 522-540; Wuenschel, P. C., The Vertical Array In Reflection Seismology—Some Experimental Studies, Geophysics, Volume 41, No. 2 (April 1976), pp. 219-232; and U.S. Pat. Nos. 4,383,308 and 4,563,757.
As discussed in greater detail in these background documents, which are hereby incorporated by reference, in principle borehole seismics involves providing a seismic source at or near the earth surface and near a borehole, and providing vertical seismic profile measurements by means of geophones positioned at selected depth levels in the borehole. The source may also be placed in a borehole which in turn could be the borehole housing the receivers. While it should be possible to position geophones at each desired depth in the borehole such that all can respond to the same seismic event generated by the source, it is believed typical to use instead a geophone (or geophones) carried by a single seismic tool which is suspended by cable in the borehole and is successively clamped to the borehole wall at selected depths, to thereby respond to different wavelets from the source at different depths.
Various kinds of seismic sources can be used, and typically it is desirable that the source produces a consistent and repeatable shot wavelet, particularly when a single downhole geophone tool is used. For example, the source can be a small chemical explosive shot near the bottom of a relatively shallow, cased and cemented well drilled near the borehole, or it can be one of the impulsive surface sources, such as weight droppers and devices that use explosive gases or compressed air to drive a heavy pad vertically downward with great force, or vibrators of the kind used as energy sources in hydrocarbon exploration. The source or sources can alternatively be placed in a neighboring well or even in the same borehole as the receivers.
The borehole can be vertical or deviated, so long as the deviation is accounted for in interpreting the measurements, and can be cased or uncased. A typical downhole tool used in vertical seismic profiling typically contains at least one geophone that is sufficiently protected to withstand the adverse environment in a deep borehole and yet can achieve satisfactory acoustic coupling with the formation. Two typical configurations are a tool that has a retractable electrically operated pivot arm which can press the geophone(s) against the borehole wall at selected depth levels, and a tool with a retractable electrically driven telescoping ram serving the same purpose. The geophone transducer element or elements in a VSP tool can be either only vertically oriented or can be, for example, in a 3-component orientation (e.g., orthogonal at xyz or tilted relative to each other at some other angle, e.g., at 54.degree.). In 3-component xyz geometry, the geophone along the z (depth) axis in a vertical borehole measures vertical particle motion, and the geophones oriented along the x and y directions measure particle motion along two orthogonal directions in the horizontal plane. Typically the three geophones are designed to exhibit closely matched amplitude and phase responses, and the device that presses the tool against the borehole wall is designed to create a geophone-to-formation bond which would result in the horizontal geophones being mechanically coupled to the formation in the same way as the vertical geophone. A 3-component tool typically also includes an orientation measuring device (typically made up of one or more magnetometers that measure azimuth from magnetic North and one or more gravity sensitive accelerometers that measure deviation from vertical), a downhole digitizing system which can digitize the geophone transducer outputs within the tool and send the digitized signals up to the surface through wires in the cable suspending the tool, and other equipment, such as devices to check the quality of acoustic coupling with the formation. Known processing equipment and techniques can be used at the surface to record the tool outputs and make preliminary corrections, such as for tool orientations, to thereby produce vector measurements which can be designated u (x=0, z, t). Each such measurement can be a digitized vector set identifying the direction in space and the magnitude of the seismic energy measured by the 3-component VSP tool at, the borehole (x=0) at depth z for each sample time t over a selected time interval. Further details can be found in U.S. Pat. No. 4,563,757.
Typically the output of any given geophone contains contribution from both compressional and shear wave components (and may contain contributions from other wave components) even when the surface seismic source is designed to optimize the generation of compressional and minimize the generation of shear waves. Even if the surface source could generate a purely compressional wave, a considerable amount of compressional wave energy may still be converted into shear wave modes whenever a propagating compressional wave encounters a reflecting surface at an oblique angle of incidence. It is believed that these converted shear wave modes can be valuable for interpreting subsurface geological conditions, as can be shear modes deliberately created by shear wave energy sources. For example, converted shear wave modes can be particularly valuable seismic measurements when used in concert with compressional wave energy measurements to interpret elastic constants of rocks or to predict the types of pore fluids in rock units or to predict other subsurface lithology parameters. In addition, certain techniques can benefit from such separation because they need, or are believed to work better with, direct or indirect measurements of only the compressional, or only the shear components of the total energy arriving at downhole geophones. One example is the use of a technique similar to medical computed tomography and relying on offset VSP, or on well-to-well VSP measurements to image the zx plane of interest. Such a technique is helped by the use of data representing the separated compressional (or perhaps shear) component of the total energy measured at the downhole geophones.
For these and other reasons, proposals have been made in the past to separate the compressional and shear wave components of the seismic energy measured at a borehole receiver. For example, the Hardage document cited earlier proposes, e.g. at page 413, that with a 3-component tool the responses of the triaxial geophone system can be mathematically rotated so that they represent the output of a single geophone oriented along the ray path of the compressional wave first arrival at each recording level, and that data can be derived which represent the response that a geophone would record if it were positioned in a vertical plane containing the compressional wave first arrival ray path and then oriented in this plane so that it is normal to the compressional wave ray path, and that these data thus would contain the full response of those downgoing shear velocity modes which travel along the same ray path as the compressional wave direct arrival, partial responses of SV modes which arrive at the triaxial geophone arrangement along ray paths that differ from the compressional wave ray path, and partial responses of later arriving downgoing or upgoing compressional wave events whose ray paths intersect the geophone assembly at various angles of inclination. The earlier cited document concerning the technical program of Sep. 11-15, 1983 in Las Vegas, Nev. proposes, e.g. at page 522, that for processing VSP data from compressional wave or shear wave sources, the apparent velocity between recording positions can be used to separate upgoing and downgoing waves, and that similarly, the P, SV, and SH modes for the direct arrival in a VSP can be isolated, based on their orthogonal polarization, but reports that both techniques break down when analyzing complex wave types such as converted waves. The same document proposes at pages 524-527 a technique which involves considering the first compressional (P) ray as included in the source-well plane, deriving a projection along the first arriving P ray, which should give mainly the first arriving P ray and following multiples, deriving a projection which is normal to that first arriving P ray and is in the source-well plane, which should give direct and converted shear SV waves, and deriving a projection normal to the source-well plane, which should give shear SH waves. The Hardage document cited earlier observes, e.g. at pages 177 and 178, that when VSP measurements taken in the space-time domain are converted to the frequency-wavenumber domain, a masking function could be superimposed over the VSP data in the frequency-wavenumber domain in order to suppress events not travelling with compressional velocity, and gives a conceptual illustration at FIG. 5-20 of a so-called pie slice velocity band pass masking function which would reduce the magnitudes of all energy modes except the upgoing compressional reflections. Other types of frequency-wavenumber velocity filtering are also discussed in the Hardage document, e.g. at pages 174-176.
The knowledge derived from VSP and/or other logs (e.g., sonic) can comprise the local compressional and shear velocities and/or the local slowness, such as the local slowness of waves in the vector wavefield. Because of the assumption that the formations adjacent the borehole are locally isotropic, there is only a single inherent P or S velocity for a given depth, and it can be assumed to be that measured by a sonic logging tool or by a zero-offset VSP. In principle, the main steps of an embodiment of the invention are to decompose the 3-component measurements into local plane wave components, identify the P and S waves of each plane wave component by polarization, and separately recombine the so-identified P and S waves.
The above and other aspects of VSP and borehole seismic methods are described in the U.S. Pat. No. 4,870,580, including further details of P- and S-wave separation and processing as may provide additional background information for the present invention.
As it is an important aspect of VSP and related borehole seismic methods to separate P- and S-wave events, VSP cables comprise typically geophones or accelerometers. Hydrophones as predominantly used in marine seismic acquisition systems, are rarely found in borehole systems. The geophones are closely coupled to the wall of the borehole via suitable springs or clamping apparatus as known in the art. If hydrophones are used in VSP equipment, these are typically combined with geophones to provide an additional pressure measurement.
In the known methods and apparatus of marine seismics, the towed streamer comprises a plurality of pressure sensitive hydrophone elements enclosed within a waterproof jacket and electrically coupled to recording equipment onboard the vessel. Each hydrophone element within the streamer is designed to convert the mechanical energy present in pressure variations surrounding the hydrophone element into electrical signals. This streamer may be divided into a number of separate sections or modules that can be decoupled from one another and that are individually waterproof. Individual streamers can be towed in parallel through the use of paravanes to create a two-dimensional array of hydrophone elements. Data buses running through each of the modules in the marine streamer carry the signal from the hydrophone elements to the recording equipment (so-called acoustic data). Hydrophones when applied in borehole seismics, have been used to measure solely a local pressure.
A hydrophone may produce electrical signals in response to variations of acoustic wave pressure across the hydrophone. Several hydrophones may be electrically coupled together to form an active section or group of an acoustic sensor array or streamer. Electrical signals from multiple hydrophones of an active section are typically combined to provide an average signal response and/or to increase the signal-to-noise ratio.
In the light of the above, it is an object of this invention to provide an improved borehole seismic acquisition system which does not require extensive clamping devices.